U.S. Pat. No. 3,697,091 relates to piston rings which have been plasma spray coated with a titanium dioxide-aluminum oxide coating wherein the coating contains from 10 to 25% by weight of titanium dioxide and the balance, aluminum oxide. U.S. Pat. No. 3,794,334 relates to piston rings which have been coated with a zirconium oxide coating. These coatings are extremely hard, and are particularly useful as piston ring facing coatings.
However, there is a production difficulty encountered in the manufacture of piston rings with this type of coating. The piston rings are normally coated by plasma spray technique, as clearly pointed out in the aforesaid U.S. patents, by mounting a number of cast iron piston ring blanks on a mandrel, and applying by plasma spray technique, a titanium dioxide-aluminum oxide or zirconium oxide coating thereon. The thickness of the coating so applied is generally in the range of between about twenty thousandths of an inch and thirty thousandths of an inch. Approximately one-half of the applied coating is removed by grinding in order to smooth and true the surface for use as a piston ring.
To effect the grinding operation, silicon carbide grinding wheels are used. One manufacturer's designation, namely Bay State Abrasive Products Company, of a typical silicon carbide grinding wheel used for this operation is IC-802-J8-V32. Because of the extreme hardness of the coatings, it has been found that the silicon carbide grinding wheel must be "in-cycle dressed" as many as five times in the course of removing such refractory metal oxide coatings to the extent of about one-half the applied depth in the finishing of a standard 4 inch piston ring.
Dressing is accomplished with a diamond dressing tool in a known manner, and each such dressing removes from the diameter of the wheel approximately 0.002 inch. The large diameter silicon carbide grinding wheels used in finishing piston rings for internal combustion engines are quite expensive, and consequently, the minimizing of dressing during the grinding cycle gives rise to a number of cost saving advantages. In the first place, since dressing necessitates removal of the surface of the wheel to present a new grinding surface, there is cost involved not only in terms of the amount of grinding wheel which is lost but also the production time required to effect dressing. Moreover, as the wheel wears, the grinding characteristics of the wheel change often necessitating adjustment in the grinding parameters.
The present invention greatly alleviates the problems in wheel dressing and the consequent loss of grinding wheel surface.
The benefits of the present invention are achieved by incorporating into the plasma powder mixture of refractory metal oxide prior to plasma application thereof a minor amount, e.g., 10 - 15% by weight, of a metal fluoride. Metal fluoride materials, such as calcium fluoride, are known and used as dry lubricants in certain ceramic oxide coatings.
Various prior art discloses that metal fluoride materials such as calcium fluoride have been used in ceramic oxide coatings. Typical of such prior art are U.S. Pat. Nos. 3,157,529, 3,121,643, and 2,869,227. Further are relating in such mixtures is the article by Hillert (Acta. Chem. Scand. 20 (1), 251-3 (1966) Eng.) and a paper by Gardos, ASLE Pre-print No. 74, LC 2C-2 entitled "Some Topographical and Tribological Characteristics of A CaF.sub.2 /BaF.sub.2 Eutectic Containing Porous Nichrome Alloy Self-Lubricating Composite" presented at the ASLE/ASME Lubrication Conference held in Montreal, Canada, Oct. 8-10, 1974. A paper presented at the same conference held in Atlanta, GA, Oct. 16-18, 1973 by Moore et al. entitled "The Friction And Wear Characteristics Of Plasma-Sprayed NiO-CaF.sub.2 In Rubbing Contact With A Ceramic Matrix" is also illustrative of related art.
None of the above noted prior art documents discloses the substantial improvement in manufacturing due to the material composition. Further, none of the art discloses the specific composition disclosed herein and none of the art is related to piston ring manufacture other than U.S. Pat. No. 3,697,091 which does not disclose the composition disclosed herein. In fact, some of the art noted discloses processes which are entirely unsuited for piston ring manufacture.